L. S. Yu

Schottky barrier engineering in III–V nitrides via the piezoelectric effect

A method for enhancing effective Schottky barrier heights in III–V nitride heterostructures based on the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/AlxGa1−xN barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess significantly larger effective barrier heights than those for AlxGa1−xN, and the influence of composition, doping, and layer thicknesses is assessed. A GaN/Al0.25Ga0.75N barrier structure optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement of 0.37 V over that for Al0.25Ga0.75N. Corresponding reductions in forward-bias current and reverse-bias leakage are observed in current–voltage measurements performed on Schottky diodes.

The role of the tunneling component in the current–voltage characteristics of metal-GaN Schottky diodes

The temperature dependence of the current–voltage characteristics of Ni–GaN Schottky barriers have been measured and analyzed. It was found that the enhanced tunneling component in the transport current of metal-GaN Schottky barrier contacts is a likely explanation for the large scatter in the measured Richardson constant.

Dependence of Ni/AlGaN Schottky barrier height on Al mole fraction

The dependence of the Schottky barrier height of Ni/AlxGa1−xN contact on the Al mole fraction up to x=0.23 was studied. The barrier heights were measured by I–V, capacitance–voltage, and the internal photoemission method. The Al mole fractions were estimated from the AlGaN band gap energies measured by photoluminescence. In the range of x<0.2 a linear relationship between the barrier height and Al mole fraction was obtained. This was consistent with the slope predicted by the Schottky rule. For x=0.23, the measured barrier height was lower than predicted. We believed this was due to crystalline defects at the Ni/AlGaN interface.

Deep level defects in n-type GaN grown by molecular beam epitaxy

Deep-level transient spectroscopy has been used to characterize electronic defects in n-type GaN grown by reactive molecular-beam epitaxy. Five deep-level electronic defects were observed, with activation energies E1=0.234±0.006, E2=0.578±0.006, E3=0.657±0.031, E4=0.961±0.026, and E5=0.240±0.012 eV. Among these, the levels labeled E1, E2, and E3 are interpreted as corresponding to deep levels previously reported in n-GaN grown by both hydride vapor-phase epitaxy and metal organic chemical vapor deposition. Levels E4 and E5 do not correspond to any previously reported defect levels, and are characterized for the first time in our studies.

A study of the Au/Ni ohmic contact on p-GaN

The formation mechanism of the ohmic Au/Ni/p-GaN contact has been investigated. We found that it is essential to (i) deposit a structure of Au and Ni in the proper deposition sequence, and (ii) anneal the bilayer structure in an oxygen containing ambient. Our findings indicated that oxygen assists the layer-reversal reactions of the metallized layers to form a structure of NiO/Au/p-GaN. The presence of oxygen during annealing appears to increase the conductivity of the p-GaN. It is further suggested that Ni removes or reduces the surface contamination of the GaN sample before or during layer reversal. In the final contact structure, an Au layer, which has a large work function, is in contact with the p-GaN substrate. The presence of Au in the entire contacting layer improves the conductivity of the contact. An ohmic formation mechanism based on our experimental results is proposed and discussed in this work.

Ni and Ti Schottky barriers on n-AlGaN grown on SiC substrates

The electrical characteristics of Ni and Ti Schottky barriers on n-Al0.15Ga0.85N on SiC were investigated. We report that the barrier height for Ni on n-Al0.15Ga0.85N was about 1.26 eV and about 1 eV or less for Ti. These barrier heights are about 0.3–0.4 eV larger than those for Ni and Ti on n-GaN, which are in good agreement with Schottky model predictions.

THERMALLY STABLE PTSI SCHOTTKY CONTACT ON N-GAN

Platinum silicide (PtSi) and Pt Schottky contacts on n-GaN have been investigated and compared. The PtSi contacts were formed on n-GaN by annealing a multilayer structure of Pt/Si with the appropriate thickness ratio at 400 °C for 1 h in forming gas. The barrier height of the as-formed PtSi contacts was found to be 0.87 eV capacitance–voltage (C–V), and remained unchanged after further annealing at 400 and 500 °C. Upon annealing at 600 °C for 1 h, the barrier height decreased to 0.74 eV (C–V), but the diodes remained well-behaved. The as-deposited Pt yielded a barrier height of 1.0 eV (C–V). Upon annealing at 400 °C for 1 h, the Pt diodes degraded and most of the diodes did not survive additional annealing at 400 °C for longer times. The electrical measurements and the Rutherford backscattering spectrometry results indicated that PtSi contacts are thermally much more stable than Pt contacts on GaN.

Study of contact formation in AlGaN/GaN heterostructures

The contact formation of Ti/Al and Ti metallization on AlGaN/GaN heterojunction field effect transistors (HFET) was investigated. It was found that ohmic contact formation is related to the low work function of the Ti contacting layer and the formation of a TiN phase at the Ti/nitride interface. Contact resistance as low as 1 Ω mm or less can be obtained on HFET samples with a nsμ product of ∼0.8×1016/V s and on n-GaN with a carrier concentration of 1.5×1018/cm3. Ti/Al bilayer contact scheme is superior to Ti-only contact due to a surface Al3Ti layer in the bilayer contact, which may reduce the oxidation problem when annealed in N2 at high temperatures. Preannealing the HFET samples at 850 °C for 1 h in N2 appears to improve the ohmic contact in general, but not always observed. Our results indicate that Ti/Al contact scheme yields sufficiently low contact resistance on HFET structures for microwave applications.

Transport properties of the advancing interface ohmic contact to AlGaN/GaN heterostructures

The transport properties of the advancing interface ohmic contact to AlGaN/GaN heterostructure field-effect transistors have been investigated. We found that carrier transport across the AlGaN barrier layer is dominated by the tunneling of electrons that originate from the two-dimensional electron gas located at the AlGaN/GaN interface. The observed temperature dependence of the specific contact resistivity is different from that of the contact on highly doped bulk semiconductors, although tunneling current dominates the carrier transport in both cases.

Ni and Ni silicide Schottky contacts on n-GaN

The electrical characteristics of Ni and NiSi Schottky diodes on n-GaN have been investigated as a function of annealing. Ni diodes were found to be stable up to 500 °C for 1 h in sequential annealing, with a barrier height φ (I–V) of 0.8–0.9 eV and an n factor of ∼1.1. The barrier height deduced from C–V measurements, φ (C–V), was typically 0.15 eV higher than φ (I–V). At 600 °C the diodes failed, and Ga was found to migrate into the Ni layer. NiSi diodes were stable up to 600 °C for 1 h, φ (I–V) was found to be about 0.8–1 eV with an n-factor of about 1.15. The value of φ (C–V) was between 0.3 to 0.6 eV higher than φ (I–V), consistent with the notion of the presence of a thin insulating layer at the NiSi/GaN interface. The electrical characteristics obtained in this study are also compared with those obtained for Pt and PtSi Schottky diodes on n-GaN.